When Rabbi Ovadia Yosef was 55 he wrote in one of his books, in a discussion about whether a girl's bat mitzvah should be celebrated like a boy's bar mitzvah, that even an ordinary birthday is important.

"Some people celebrate the day of birth every year, it's a nice custom and one we keep in hour house," Yosef wrote. The edict was not revolutionary but carried the accuracy, expertise and freshness characterized in many of his answers on halakhic issues.

Next Monday Yosef, a senior halakhic authority and one of the most important political leaders in recent generations, will turn 90 years old. Biographies, books based on his teachings, doctorate papers and newspaper articles have crowned him in Israel and abroad with numerous superlatives. Perhaps more than anyone else, Yosef has influenced and shaped the state in his position as rabbi and spiritual leader of the largest Haredi political party. Heads of state don't make frequent pilgrimages to every bearded man with "rabbi" on his door, after all.

Next week during the holiday of Sukkot, President Shimon Peres, who recently celebrated his 87th birthday, will visit the Baghdad-born Yosef, who is a former chief rabbi of Israel, Israel Prize laureate, Shas' founder, spiritual mentor and the author of dozens of books.

Three weeks ago the rabbi showed he could still raise hackles with a single utterance. He wished a fatal plague on the Palestinians (whom he called the "Ishmaelites" ) and their leader Mahmoud Abbas. His words evoked condemnations from Washington, Jerusalem and Ramallah, to name a few, and embarrassed all those who mistook him for a pacifist. Thirty-one years ago, he ruled the territories should be evacuated in a peace agreement.

One of his relatives said "five minutes after he spoke he felt deep sorrow and looked for a way to mend it." Yosef's scandalous statements over the years have raised questions over whether he is a halakha expert or a political leader. He has ranted, raved and hurled curses at Shulamit Aloni, Ariel Sharon, Netanyahu, Yossi Sarid, Abbas, the secular public and Holocaust victims. His outrageous outbursts have antagonized the secular public.

Yosef's close followers say that as time goes by, his halakhic project, intended to restore Sephardi rabbi Yosef Karo's title as the halakhic leader of all Sephardi Jews, has taken an increasingly important role in his life.

Yosef wants his books to be in every library next to Maimonides' Mishneh Torah and Karo's Shulhan Aruch, his confidants say. He thinks history will forget everything but his rabbinic writings.

"Precisely because of his birthday the rabbi has a strong feeling that time must not be wasted," one of his confidants says. "Every day beyond 80 is a gift, he says. That's why he's writing all the time, tirelessly."

Yosef attributes no importance to newspaper headlines. But his statement three weeks ago touched a sensitive spot in Yosef's relations with the non-Hasidic ultra-Orthodox world. The newspaper Yated Neeman published an article by Lithuanian community leader Rabbi Yosef Shalom Elyashiv, denouncing Yosef without mentioning him by name. It accused him of violating a basic value in the ultra-Orthodox approach to exile that prohibits "provoking nations."

Yosef and Elyashiv, who is 10 years his senior, have a complex relationship. In the 1950s they both belonged to an elite group of scholars and later served together on the Great Rabbinical Court.

In most ideological-halakhic issues over the past years Yosef has retracted his initial position, or kept silent in the face of Elyashiv's response. When Yisrael Beiteinu sponsored a bill facilitating conversion, Yosef first co-drafted the bill. But after the Lithuanian Haredi community attacked it, Shas followed suit and objected to it. Yosef was at first in favor of legislation encouraging organ donation in case of brain death, but reneged when the Lithuanians criticed it.

Yosef has remained silent in the saga of moving the graves at Barzilai Medical Center and in the violent demonstrations of the Eda Haredit, a prominent ultra-Orthodox Jewish communal organization. He also didn't speak out about the mother who starved her infant son and the attack on Interior Minister and Shas head Eli Yishai in Jerusalem's Mea She'arim neighborhood.

Many of his supporters questioned his silence over the discrimination of Sephardi girls in Immanuel, wondering why he did not support the Sephardi parents.

At 90, Yosef is not a healthy man, but he intends to fast on Yom Kippur. He lives with his young son Moshe, his wife Yehudit and their children, and is under close supervision of his personal doctor. He eats little but refuses to give up pickled eggplants from the Mahane Yehuda market. He also listens to a Farid Al Atrash disc occasionally.

Haaretz.com, the online edition of Haaretz Newspaper in Israel, and analysis from Israel and the Middle East. Haaretz.com provides extensive and in-depth coverage of Israel, the Jewish World and the Middle East, including defense, diplomacy, the Arab-Israeli conflict, the peace process, Israeli politics, Jerusalem affairs, international relations, Iran, Iraq, Syria, Lebanon, the Palestinian Authority, the West Bank and the Gaza Strip, the Israeli business world and Jewish life in Israel and the Diaspora.